As the dental practice advances with the discovery of new filling materials, there developed a need for a new system to place such materials in a tooth. The earliest known delivery system for use with composite resin type dental materials is disclosed in U.S. Pat. No. 3,581,399 issued to Dragan on June 1, 1971. Therein disclosed is a manual extruder for positioning viscous dental material within a prepared tooth. A further improved delivery system is disclosed in U.S. Pat. No. 4,198,756 to Dragan dated April 22, 1980. The device disclosed therein provides a mechanical advantage for the controlled dispensing of the viscous dental material. These devices permit improved dental filling techniques, in that the viscous material is placed in the tooth cavity from the bottom up; to result in the elimination of voids in the tooth filling, which occurred in the previous technique of using a spatula to pack the viscous dental material from the exterior to the interior of the tooth.
As might be expected, refinements have been made to the initial concepts disclosed in the above mentioned Dragan patents. One such refinement is disclosed in U.S. Pat. No. 4,295,828 issuing to Rudler on Oct. 20, 1981. Therein disclosed is a manual extruder very similar to the initial Dragan U.S. Pat. No. 4,198,756, but differing therefrom in that the front end of the barrel is provided with a hinging section for loading and locking a cartridge in place at the front end of the barrel.
It was recognized early on that a syringe having a barrel construction with a snap fit front end construction would facilitate the positioning of a cartridge to the front end so as to provide for easy and rapid placement of a cartridge in the syringe device. Such snap fit front end barrel constructions are disclosed in U.S. Pat. Nos. 3,076,455 and 3,220,412 to McConnaughey dated Feb. 5, 1963 and November 1965 respectively. McConnaughey teaches a syringe device with a snap fit to hold a cartridge onto the end of a barrel by lateral movement relative to a side opening that is accomplished by momentarily distorting the holder to widen the side opening through which the cartridge is inserted to provide the pressed or snap fit. McConnaughey, in the U.S. Pat. No. 3,076,455, FIG. 12 thereof, also discloses a snap fit channel 80 having an internal groove or depression 82 for receiving the flange or bead 78 of the cartridge to retain the cartridge at the front end of a barrel portion and to prevent axial displacement of the cartridge. Brazilian patent application MU5701465 filed Nov. 22, 1977 and published July 3, 1979 discloses a similar "snap or pressed" fit front end construction. The snap fit with undercut groove for retaining a cartridge as applied to a manual extruder for dental materials are thus well known. Such snap fit front end barrel constructions have also been utilized in other dental syringe devices as disclosed in U.S. Pat. No. 4,330,280 and U.S. Pat. No. 4,384,853. These latter two U.S. patents disclose a manually operable dental syringe for cartridge containing dental material similar to the dental syringe of U.S. Pat. No. 4,198,756, but utilizing a front end barrel construction having a snap fit as suggested by McConnaughey. The snap fit construction disclosed in said latter two patents is provided with an undercut groove to receive the flanged end of the cartridge wherein the sidewalls of the groove at the upper edges have limited flexibility and spaced apart slightly less than the diameter of the cartridge flange to effect a limited snap connection of the cartridge flange within the undercut groove of the holder.
While these modifications attempted to facilitate the easy placement of a cartridge in a manual extruder, problems with respect thereto have been noted. For example, the inherent flexibility necessary to provide for a snap fit to facilitate attachment of a cartridge, e.g. as disclosed in U.S. Pat. Nos. 4,330,280 and 4,384,853, results in the susceptibility of the side walls of the snap fit compartment or groove to be forcibly spread apart during use if an excessive pressure is applied during an extruding operation. This may well cause the cartridge to be literally shot from the barrel during an extruding operation. This is particularly troublesome when very high extruding pressures are required to be applied to extrude the viscous dental material necessary with such dental cartridges. This is caused by the flange of the cartridge acting as a wedge to force the flexible sidewalls of the snap fit front end to unintentionally spread apart sufficient to cause the cartridge to be forced from its compartment or groove. This is particularly undesirable in a dental procedure when the cartridge at the front end of the syringe is placed in the oral cavity. Should such cartridge be dislodged from the syringe, it can be accidentally inhaled or swallowed by the patient, thereby causing serious harm to the patient. Also, wear which normally occurs in use may also cause the cartridge to become loosely held by the syringe to result in unintentional separation of the cartridge during an extruding operation.